Nickel plating process



United States Patent 3,357,854 NICKEL PLATING PROCESS Darrell D. Hays, Kennewick, Wash., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Oct. 30, 1964, Ser. No. 407,940 13 Claims. (Cl. 117-130) ABSTRACT OF THE DISCLOSURE An electroless process and bath for plating metal articles with nickel containing a water-soluble nickel salt, a

hypophosphite and a trace amount of inorganic alkali metabisulfite in an aqueous solution.

This invention deals with a nickel plating process, and in particular with a process of plating nickel on metal bases by a so-called chemical or electroless method, where neither an electric current nor electrodes are used, from a bath containing a dissolved nickel salt and alkali metal hypophosphite. While the process is primarily intended for the plating on aluminum metal and alloys, it is also suitable for other base metals, for instance iron, steel, copper or brass.

Nickel-plated metals have a great many uses in the industry. One application currently important is that for fuel elements in nuclear reactors, for instance for aluminum-clad uranium fuel elements.

Aluminum-base metals have been nickel plated heretofore by electroless processes. Such processes previously used are described, for instance, in an article by Abner Brenner in Metal Finishing, 52, November 1954, pages 68-76, and December 1954, pages 61-68, and also in an article by Gregoire Gutzeit in Metal Progress, 66, July 1954, pages 113-120.

The processes there described have one great drawback: they proceed at a rather slow rate, which makes them comparatively uneconomical.

It is an object of this invention to provide a process of plating metals with nickel by an electroless method which yields a satisfactorily thick deposit in a relatively short time.

It is also an object of this invention to provide a process of plating aluminum-clad fuel elements for nuclear reactors with nickel whereby a coating is obtained that does not gall when the fuel element is moved in the reactor.

It is another object of this invention to provide a process of plating metals with nickel whereby a nonporous coating is obtained that has good corrosion resistance at elevated temperature and protects the underlying metal base well against corrosion.

It is still another object of this invention to provide a process for plating nickel on metals in which the thickness of the deposit can be controlled easily and accurately, regardless of the geometry of the article.

It is finally also an object of this invention to provide a process for plating nickel on metal bases whereby a coating is obtained that adheres excellently to the metal base and which does not blister, even when exposed to high neutron fluxes.

It has been found that, if a trace quantity of an alkali metabisulfite, for instance Na S O is incorporated in a plating bath containing a water-soluble nickel salt and hypophosphite, the plating rate is considerably increased. No explanation can be given for this phenomenon.

The process of this invention comprises adding a trace of an inorganic alkali metabisulfite to an aqueous solution of a nickel salt and a hypophosphite; immersing a metal article to be plated, whereby nickel metal is deposited on said article; and replenishing the metabisulfite content as it is depleted and the plating rate diminishes, until a deposit of the desired thickness has been obtained.

Any water-soluble nickel salt can be used for the process of this invention; for instance, nickel chloride and nickel sulfate have been found suitable. The concentration of the nickel salt may vary widely; good results were obtained with a concentration of about 30 grams per liter.

The hypophosphite serves as a reducing agent for the nickel salt and its concentration may also vary. Sodium hypophosphite in a concentration of about 10 grams per liter was found satisfactory.

While the above concentrations of the plating bath were not critical, the concentration of the inorganic metabisulfite is critical, as will be shown later in the example. The sodium metabisulfite was the preferred salt; the concentration should range between 10- and 10 mole per liter. With this concentration the plating rate was increased by from 45 to 60% over the rate without the metabisulfite. The deposition rate is the indicator for the depletion of the metabisulfite concentration and the necessity of further additions thereof; it therefore should be observed closely. Whenever the deposition of nickel metal slows down noticeably, a further installment of metabisulfite should be added.

Organic carboxylic acids or their water-soluble salts are advantageously added as buffers and stabilizers of the bath as they have also been used in the conventional electroless nickel plating processes. Carboxylic acids suitable for this purpose are succinic acid, citric acid, tartaric acid, acetic acid and hydroxyacetic acid; however, lactic acid was preferred and advantageously used in a concentration of between 15 and 40 grams per liter.

The process of this invention can be carried out from an acid or an alkaline plating solution, but the acid bath is preferred. Typical bath compositions, for instance for the alkaline and acid bath, respectively, are: 30 g./l. Ni'Cl -6H O, 50 g./l. NH Cl and 10 g./1. NaH PO -H O (pH 810) and 30 g./l. NiSO -7H O, 10 g./l. NaH PO -H O and 15-40 g./l. lactic acid (pH 4-5).

During operation the acidity of the bath increases. In order to keep the pH value at a rather constant level, small quantities of ammonium hydroxide or sodium hydroxide were added from time to time as needed.

The process can be carried out at room temperature, but elevated temperature is preferred. A temperature of about C. yielded the best results.

It is advantageous to agitate the bath during deposition in order to maintain a uniform bath composition at all points of the bath and at all times and thus to obtain a uniform metal deposit. Agitation also removes any hydrogen that may deposit on the surface of the metal article.

As indicated before, the concentrations of the bath have to be monitored continuously and be replenished with hypophosphite, nickel salt, metabisulfite and alkali hydroxide. From time to time the bath has to be regenerated by ion exchange for removal of the phosphite ions formed and by cooling for the precipitation of the sodium sulfate.

Under the conditions set forth the plating rate usually exceeds 1 mil per hour and frequently was found to be as high as 1.8 mil per hour, which compares favorably Example A sample of aluminum metal was immersed at room temperature in an aqueous solution 0.3 M in sodium hypophosphite, 0.1 M in nickel sulfate and 0.3 M in lactic acid. Nickel was deposited at a rate of 0.6 mil per hour.

Sodium metabisulfite was then added to the bath to yield a concentration of M, by which the deposition rate was increased to 0.95 mil per hour.

When a second installment of 10 M of sodium metabisulfite per liter was added to the above plating bath containing metabisulfite in a concentration of 10- M, the plating stopped. After allowing the system to stand at room temperature for about hours, the plating process started again and proceeded at an average rate of 1.02 mils per hour for about an hour. After this hour, the plating rate had decreased, however, to 0.75 mil per hour. At this point sodium metabisulfite was again added in a quantity of 10* mole per liter. This brought the plating rate up to 1.3 mils per hour.

Again, after a while, the plating rate decreased and, when it had reached 0.84 mil per hour, 10' M of sodium metabisulfite was added per liter. This increased the plating rate to 1.29 mils per hour.

The above example clearly ence of the metabisulfite if it not above 10' mole per liter.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process of plating a metal article with nickel, comprising adding 10" to 10- mole per liter of an inorganic metabisulfite to an aqueous solution containing a watersoluble nickel salt and a hypophosphite in an amount sufficient to reduce said nickel salt; immersing said metal article in said solution, whereby nickel metal is deposited thereon; and replenishing the metasulfite content to the initial concentration as it is depleted and the plating rate diminishes, until a deposit of the desired thickness is obtained.

2. The process of claim 1 wherein the aqueous solution contains about grams per liter of a water-soluble nickel salt and about 10 grams per liter of sodium hypophosphite.

3. The process of claim 1 temperature of about 95 C.

shows the beneficial influis present in concentration wherein the solution has a 4. The process of claim 3 wherein the solution also contains lactic acid.

5. The process of claim 4 wherein the lactic acid is present in a concentration of between 15 and 40 grams per liter.

6. A process of plating a metal article with nickel, comprising adding from 10* to 10- mole per liter of sodium metabisulfite to an aqueous solution containing 30 grams per liter of a water-soluble nickel salt, about 10 grams per liter of sodium hypophosphite and 15 to 40 grams per liter of lactic acid; heating to C.; immersing said metal article into said solution whereby nickel metal is deposited thereon; and replenishing the metabisulfite content to the initial concentration as it is depleted and the plating rate diminishes, until a deposit of the desired thickness is obtained.

7. A bath for electroless nickel plating, comprising an aqueous solution containing a water-soluble nickel salt, a hypophosphite in an amount sufficient to reduce said nickel salt and an inorganic alkali metabisulfite in a concentration of between 10' and 10' mole per liter.

8. The bath of claim 7 wherein the aqueous solution contains about 30 grams per liter of the nickel salt and about 10 grams per liter of sodium hypophosphite.

9. The bath of claim 7 wherein the aqueous solution also contains lactic acid.

10. The bath of claim 9 wherein the lactic acid is present in a concentration of between 15 and 40 grams per iter.

11. A bath for electroless nickel plating, comprising an aqueous solution of 30 grams per liter of a water-soluble nickel salt; 10 grams per liter of sodium hypophosphite; from 15 to 40 grams per liter of lactic acid; and from 10'' to 10- mole per liter of sodium metabisulfite.

12. An aqueous bath for electroless nickel plating containing 30 grams per liter of NiCl -6H O; 50 grams per liter of NH Cl; grams per liter of sodium citrate; 10 grams per liter of NaH PO and from 10" to 10' mole per liter of sodium metabisulfite.

13. An aqueous bath for electroless nickel plating containing 30 grams per liter of NiSO -7H O; 10 grams per liter OfNaI-I PO -H O; 15 to 40 grams per liter of lactic acid; and from l0 to 10* mole per liter of sodium metabisulfite. References Cited UNITED STATES PATENTS 2,762,723 9/1956 Talmey et al 117130 2,956,900 10/1960 Carlson et a1 117-130 X RALPH S. KENDALL, Primary Examiner. 

1. A PROCESS OF PLATING A METAL ARTICLE WITH NICKEL, COMPRISING ADDING 10-7 TO 10-3 MOLE PER LITER OF AN INORGANIC METABISULFITE TO AN AQUEOUS SOLUTION CONTAINING A WATERSOLUBLE NICKEL SALT AND A HYPOPHOSPHITE IN AN AMOUNT SUFFICIENT TO REDUCE SAID NICKEL SALT; IMMERSING SAID METAL ARTICLE IN SAID SOLUTION, WHEREBY NICKEL METAL IS DEPOSITED THEREON; AND REPLENISHING THE METALSULFITE CONTENT TO THE INITIAL CONCENTRATION AS IT IS DEPLETED AND THE PLATING RATE DIMINISHES, UNTIL A DEPOSIT OF THE DESIRED THICKNESS IS OBTAINED. 